Frequency measuring instrument



Aug. 25, 1959 1 W* MQTZ ET AL 2,901,699

FREQUENCY MEASURING INSTRUMENT Filed April 19, 1957 2 Sheets-Sheet 1 E5N NNN ug. 25, 1959 1 W, MOTZ ET AL 2,901,699

FREQUENCY MEASURING INSTRUMENT Filed April 19, 1957 2 Sheets-Sheet 2INVENTOR A MS *SM yUnia-2d States arent G FREQUENCY MEASURINGINSTRUNIENT Joseph W. Motz, Silver Spring, Md., and John H. Reaves,McLean, Va.

Application April 19, 1957, Serial No. 653,923

Claims. (Cl. 324-78) This invention relates to frequency measurementsand particularly contemplate a novel apparatus and method by which thefrequency of an unknown signal or sound can be instantaneouslydetermined by comparing the period of the signal to be measured with acalibrated time interval which can be varied to cover all periods in aspecified frequency range.

An immediate object of this invention is to provide a frequencymeasuring apparatus which is capable of measuring the instantaneousperiod of an unknown signal.

Another object of this invention is to provide a frequency measuringapparatus which is capable of determining the frequency of an unknownsignal by comparing its period with that of a time-Calibrating signal.

A further object of this invention is to provide a frequency measuringdevice in which the fundamental frequency of a complex signal such as asound source can readily be determined.

It is a still further object of this invention to provide a frequencymeasuring device which is particularly suitable for use in connectionwith the tuning of musical instruments, and in testing or Calibratingany periodic phenomenon.

An additional object of this invention is to provide a frequencymeasuring instrument which can accurately measure the frequency oftransient signals.

Further objects of this invention will become apparent as thedescription proceeds. One embodiment of the in- Vention is illustratedin the accompanying drawings in which:

Fig. 1 is a block diagram of a particular embodiment of the invention;

Fig. 2 is a circuit schematic detailing the construction of the timeCalibrating portion of the apparatus shown in Fig. 1;

Fig. 3 shows the circuit construction of a time interval comparatoremployed in the embodiment of Fig. 1 and Fig. 4 is a circuit diagram ofa differentiator and pulsepolarity selector employed in connection withthe present invention.

The present invention provides means for determining the fundamentalfrequency of an unknown signal by comparing the period of thefundamental frequency contained in the unknown signal with a preciselydefined time period. The invention also provides means for effectingsuch comparison over a particular frequency range commensurate with thefrequency range of the unknown signal.

Time interval comparison in accordance with the principles of thisinvention is accomplished by converting the wave-form of an unknownsignal into a symmetrical square wave having a period equal to amultiple, for example two, of the unknown signal. The first transitionof each cycle of such square-wave is then employed to generate arectangular wave having a period equal to the square wave but containingan adjustable time interval.

`Means are provided for Calibrating the time interval of the rectangularwave, and such time interval may then be compared with the half-periodof the square wave by means of a special bridge circuit forming part ofthe invention, When the bridge is balanced, the period of the testsignal will be equal to the calibrated time interval. The frequency ofthe unknown signal is thereby determined.

In order to cover a particular frequency range, a frequency divisionsystem preferably employing a series of binary scalers may be used. lfthe lowest frequency to be measured has a period equal to to, then thecalibrated time interval of the referred-to rectangular wave must coveronly the range from t0 to For all frequencies greater than twice thelowest such frequency, the binary scalers convert the frequencies sothat they are kept within the range covered by that of the referred-torectangular wave.

One embodiment of the invention is schematically illustrated in Fig. 1of the drawings. The unknown, or test signal, the frequency of which isto be measured is indicated in Fig. l as emanating from any signalfrequency source which may be a sound source for purposes ofillustration. The test signal is initially detected by a transducer suchas the microphone M, The signal is then amplified in an amplifier i andapplied to a rectangular wave generator 2. The waveforms of the signalsin the various circuit components are symbolically illustrated alongsideeach component in Fig. l. Since the invention is singularly adapted tomeasure frequencies of transient signals such as periodic waveforms theamplitudes of which may decay with time, the amplifier l is chosen sothat its output ampiitude is indepedent of the amplitude of the inputsignal. Any consequent clipping of the signal is immaterial since thewaveform of the test signal does not affect the operation of thesubsequent portions of the circuit.

The rectangular wave generator 2 may be of a conventional type such asdescribed, for example, on page 204 (Fig. 4l.a) of ElectronicExperimental Techniques by Elmore and Sands. It will be noted in Fig. lthat the recurrence period T of the rectangular waves generated bygenerator 2 corresponds to that of the test signal being measured.

A differentiator and pulse-polarity selector circuit 3 receives theoutput of rectangular wave generator 2 and converts the trailing edgesof the rectangular waves into sharpened pulses of like repetition periodT and having a polarity as indicated in Fig. l. The construction of thedifferentiator and pulse-polarity selector is detailed in Fig. 4 to bedescribed.

The referred-to differentiated pulses are then employed to trigger ascale-of-two scaler 4 which may be of the type described on page 216 ofthe above referred-to text. The output of the scale-of-two circuit tthen comprises a repetitions square wave signal each cycle of which hasa duration or half-period corresponding to the period T. In this mannerthe repetition period of the particular frequency being analyzed ismanifested as the halfperiod of a symmetrical square wave.

The output signal from sealer 4 may then be applied to a series ofscale-of-two sealer circuits 5a, 5b, 5c, 5d and 5e of similarconstruction as the sealer 4. The purpose of the scaling circuits 5er-5eis to scale down the converted test signal to a particular sub-multiplecorresponding to that of a standard Calibrating source to be described.It will be understood that the number of such scaling units shown inFig. 1 is illustrative and any convenient number can be employedcommensurate with the frequency range of the unknown or test signal. For

example, when the invention is employed in connection wth the tuning ofmusical instruments, each scale-of-two circuit a etc., can be made tocorrespond to a separate octave of the musical scale. The output of eachscale of two circuit is applied, respectively, to respective contacts ofa switch SW1. The contactor of the switch SW1 is connected to a seconddiiferentiator and pulse-polarity selector 6 and to a timeinterval'comparator bridge 9 to be described. The pulse-polarityselector 6Vis of the same type as the corresponding member 3. The outputfrom each of the scaler units 4, 5a etc., as manifested on each of thecontacts of switch SW1 is symbolized in Fig. l by the square waverepresentation having the indicated half-period nT where n correspondsto the division factor provided by the particular scaler unit selectedby switch SW1.

The ditferentiator and pulse-polarity selector 6 provides sharpenedpulses having a rate of recurrence of 2nT.

These pulses are applied through an inverter 6a to a conin the bridgecircuit 9 by means of the balance indicator 9a. The time calibratingcircuit diagrammatically shown in Fig. l may preferably be specicallyembodied in the circuit ydetailed in Fig. 2. The over-al1 operation ofthe referred-to time Calibrating mechanism will first be described byreference to Fig. l.

The linear sweep generator is a saw-tooth Wave generator the sweep-rateof which is determined by an RC circuit comprising the frequencycalibrator control R and a selected one of the capacitors C. Any one ofthe capacitors designated as C can be selectively integrated into thesweep circuit by means of selector switch SW2. The capacitor areillustrated in Fig. 1 as correspondingrto the lowest octave of a pianokeyboard to indicate the manner in which the apparatus of this inventioncan be employed to tune musical instruments. It will be readily apparenthowever that the particular frequency selected can option- `allycorrespond to that of any signal to be analyzed since the invention snot limited to the frequency range of a musical scale.

The time calibrator further includes a voltage discrimmined by thereferred-to RC value. A linear sweep signal of the type symbollizedadjacent the linear sweep generator 8 in Fig. l is established in suchdescribed manner.

The cascade connected amplifier tubes V5, V6, together with thefeed-back circuit from the plate of V6 to the grid of tube V5 insurelinearity of the sweep signal. Specically, the voltage acrossV theselected capacitor C is ampliied by V5 and V6 which have a combined gainof unity and the signal is fed back through resistors R1 and R2.Discharge of the capacitor C is accomplished through sweep control tubeV4.

As indicated in Fig. l, the linear sweep signal generated by sweepvgeneratorV 3 is timed to occur in synchronism with the square-wavesignals obtained from frequency divider stages 4, 5a etc. The bi-stablecircuit comprising tubes V1 and V2 of control gate generator 7 in Fig. 2provides for such synchronism. The bi-stable circuit comprises a. pairof twin triodes connected as indicated. The sharpened pulses fromditferentiator 6 are inverted in inverter 6a and, as indicated in Fig.l, have a repetition Vrate of ZnT. These pulses. are applied to the gridof tube V1. Since the grid of control tube V4 is connected to the outputof tube V1, when the later is Vrendered conducting by the applied signalfrom differentiator 6, tube V4 will lbe cut ot and the selectedcapacitor C will be Charged from the plus lSO-Volt source as described.`The right-hand section of tube V2 is employed as a voltagediscriminator. That is, the voltage across the selected capacitor C isapplied through cathode follower V3 to inator 10 which limits theamplitude ofthe 'saw-tooth i' wave generated by linear sweep generator8, by passing only the portion of the signal which exceeds the amplitudeV indicated in the waveform to the left of voltage discriminator 1t) inFig. l. The voltage discriminator 10, as will be described, conductswhen the amplitude of the input signal applied thereto exceeds the valueV. When thus energized, the voltage discriminator applies a signal tothe control gate generator 7, which immediately resets the sweepgenerator 8 to terminate the sweep.

Fig. 2 shows a particular implementation of the time Calibrating circuitcomprising the elements 7, 8 and 10 in Fig. l. The portions of thecircuit shown in Fig. 2 corresponding to the respective blocks 7, 8 and10 in Fig. 1 bear like designations in Fig. 2. Y

The linear sweep generator (element S in Fig. fl) comprises the circuitincluding tubes V4, V5, V6, resistors .R1 and VR2 and capacitors C inFig. 2. Tube V4, when rendered conducting, provides a discharge path forthe particular capacitor C selected by vswitch SW2. The capacitors Ccorrespond to the like `designated capacitors indicated in Fig. l. Thecapacitors C together with adjustable resistor R1 and R2 comprise an RCor charging circuit, the period of which is dened by the particularvalue of the capacitance and resistance selected. When tube V4 is cutoit, the selected capacitor -C will becharged VAfrom the indicated plusISO-volt source at a rate deterthe right-hand grid of tube V2. It willbe apparent that the voltage across the capacitors must reach apredeteryruined level, corresponding, for example, to the amplitude Vindicated in Fig. l, before tube V2 will conduct. As soon as suchamplitude level is reached, the consequent conduction of the right-handsection of tube V2 functions in a characteristic manner to reset tube V1of the bistable circuit. When tube V1 is cut oft, the resulting positivesignal applied to the grid of sweep control tube V4 renders the latterconducting to produce instantaneous discharge of the selected capacitorC thus terminating the sweep signal. In this manner. the application ofeach triggering pulse from pulse-polarity selector 6 initiates a linearsweep having an amplitude V and a period corresponding to t. i

Referring to Fig. l, it will be apparent from the above description,that the selected capacitor C results in the selection of a timeinterval which determines the reference period for frequency comparison.

The construction and operation of the Vtime interval comparator 9 inFig. l will be apparent by reference to Fig. 3. The comparator comprisesseparate signal ampliers such as each section of the twin triode V7 anda null-balance meter 9a connected across the anodes. 'The plate loadresilstors R3, R4 are of equal value. The test signal input applied tothe left-hand section of tube V7 correspond tothe square-wave outputsignal .derived from the selected one of the scale-of-two circuits 4, 5aetc., as previously described, whereas the krectangular wave timecalibrating signal from the time Calibrating circuit (Fig. 2) is appliedto the grid of the right-hand section. lt will therefore be apparent,thatV the meter will be in balance only when the period nfl which is amultiple of the test signal period is equal to the calibrated timeinterval t.

The construction of the previously referred-to differentiator vandpulse-polarity selectors 3 and 6 (Fig. 1) is detailed in Fig. V4. Suchdevices comprise a diode V8 shunted by a resistor R5. A .capacitor C4Visconnected in series between the input terminal 2a and the anode of thediode. The difterentiator and pulse-polarity selectors 3 and 6, functionas follows. Application ofthe rectangular waves from either generatori?.or scale-of-two circuits 4a etc., across the resistor .R5 and capacitorC4 results in'diierentiation of the rectangular wave. Because of therectifying action of v.diode V8, .only the negative `diierentiated pulseis utilized as indicated by the output wave `forms adjacent the pulseselector 3 in Fig. 1.

The operation of the apparatus of this invention will be apparent fromthe above description. Briefly, a test signal to be analyzed, which maybe in the form of a sound source, is applied to the microphone M and isconverted into a rectangular wave having a repetition period Tcorresponding to that ef the test signal by rectangular generator 2. Thesignal is then converted into a series of negative pulses having a likerepetition period T and these pulses in turn energize a scale-of-twomulti-vibrator 4. The resulting output signal is a square wave, having ahalf-period T corresponding to that of the test signal.

The scaler circuits S11-5e provide suitable time division of the squarewave signal into any desired submultiple. The square wave is thendifferentiated and employed to trigger'the linear sweep generator 8f.V eThe sweep generator produces a wave which rises linearly with time.Since the amplitude of such wave is determined by discriminator 1th asdescribed, there results a precise time calibrated signal having aduration t. The time interval comparator 9 is then employed to comparethe time Calibrating signal with the half-period of the square(reference) wave. Since the reference signal initiates the mechanismwhich generates the time-Calibrating signal, it will be apparent fromthe above description that the reference, and time Calibrating signalsare always in synchronism.

The apparatus of the present invention is particularly suitable for usein measuring the frequencies of transient signals. In particular, aperiodic Wave form such as is represented in a signal resulitng when thekey of a piano is struck, has a variable or decaying amplitude waveform.As is evident from the above description, energization of the generator2 by amplifier 1 for only one period of the test signal is sutiicient toeffect a frequency determination.

While a preferred embodiment of the present invention has beendisclosed, it will be understood that various modilications may be madein the specific construction illustrated and described without departingfrom the spirit or scope of the invention. It is not desired to belimited other than by the scope of the claims which follow.

What is claimed is:

1. Apparatus for measuring the instantaneous frequency of a periodic,recurring unknown signal comprising: means responsive to the unknownsignal to be measured for generating a reference signal of predeterminedwave-form having a duration corresponding to a function of the period ofsaid signal, means responsive to said reference signal for generating atime Calibrating signal having an adjustable, predetermined time periodwhich can be varied means responsive to said time Calibrating signalgenerating means for Converting said time calibrating signal into asignal having a wave-form corresponding to that of said reference signaland means for comparing the respective durations of said reference, andsaid converted time Calibrating signals.

2. The invention of claim 1 in which said reference signal generatingmeans comprises a rectangular wave generator for generating arectangular wave having a duration corresponding to a function of theperiod of said signal and said time Calibrating signal generating meanscomprises means for generating a constant-slope ramp signal voltagewhich increases linearly with time and amplitude sensitive means forterminating the duration of said latter signal.

3. Apparatus for measuring the instantaneous frequency of a periodicallyrecurring unknown signal Comprising: means responsive to the unknownsignal to be measured for generating a reference rectangular wave signalhaving a duration corresponding to the period of said signal, frequencydividing means responsive to said reference signal generating means forconverting said reference signal into a square wave signal having aduration which is a multiple of said period, means responsive to saidfrequency dividing means for generating an adjustable rectangular timeCalibrating signal of predetermined duration and means for comparing theperiods of said converted reference signal and said time Calibratingsignal respectively.

4. The invention ofV claim 3 in which said time Calibrating signalgenerating means comprises means for generating a Constant-slope rampsignal which increases linearly with time and means responsive to theoccurrence of said converted reference signal for energizing saidlinearly increasing signal generating means.

5. The invention of claim 4 in which said linearly increasing signalgenerating means includes means for selectively varying the period ofsaid linear sweep.

6. The invention of claim 3 in which said frequency dividing meanscomprises a series of cascaded scale-oftwo counters and means forselectively connecting said linearly increasing signal generating meansto one of said Counters.

7. The invention of claim 3 in which said comparing means comprises abridge circuit.

8. The invention of claim 5 in which said means for varying the periodof said linearly increasing signal Comprises a plurality of impedancemeans providing a range of adjustment corresponding to a musical scale.

9. The invention of claim 1 in which said time calibrating signalgenerating means comprises a pulse generator for periodically generatinga gating pulse in response to the applied reference signals, a linearlyincreasing signal generator energized by said gating pulses and meansfor terminating the duration of said linearly increasing signal.

10. A device for measuring the instantaneous frequency of a periodicallyrecurring test signal comprising: means for converting said test signalinto a symmetrical square 'wave having a period equal to a multiple ofthe period of said test signal, means responsive to said Convertingmeans for generating a linearly increasing signal, means for adjustingthe duration of said linearly increasing signal, means responsive tosaid linearly increasing signal generating means for generating arectangular-wave signal having a duration determined by said linearlyincreasing signal and means for comparing the duration of saidrectangular signal with the half-period of said square wave.

References Cited in the tile of this patent UNITED STATES PATENTS2,732,496 Slonczewski Ian. 24, 1956 2,756,336 Christensen July 24, 19562,806,953 Krauss Sept. 17, 1957

